Use stack buffers for Retry-After headers

[RAprogramm]

Summary

• avoid allocating when emitting Retry-After in the axum and actix integrations by formatting into stack buffers
• extend the feature-gated adapter tests to assert the ASCII header values remain intact

Testing

• cargo build --all-targets
• cargo clippy -- -D warnings
• cargo test --all
• cargo test --all --features "axum actix"
• cargo doc --no-deps
• cargo audit
• cargo deny check

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https://chatgpt.com/codex/tasks/task_e_68d778d705f0832bb9ed7df8df45feb4